During engine emissions testing, an exhaust catalyst may be required to be reach a desired conversion efficiency within a predetermined time following an engine start from cooled-to-ambient conditions. For example, in some engine emissions testing, the exhaust catalyst may be required to operate at greater than 80% efficiency within 20 seconds following an engine start. Exhaust catalyst operating temperatures may begin to operate with such efficiency in temperature conditions ranging from approximately 800 to 1600 degrees Fahrenheit, depending on the composition catalyst. Thus, in order to increase the efficiency of an exhaust catalyst within a short period of time following an engine start, various operational strategies may be employed to heat the catalyst and expedite the light-off of the exhaust catalyst.
Attempts to address issues related to expediting the light-off of a catalyst following an engine start may include adjusting an ignition timing or an air to fuel ratio (AFR) of the engine to increase an exhaust gas temperature. However, the inventors have recognized that while adjusting ignition timing and AFR attempts to expedite catalyst light-off by increasing an exhaust gas temperature, these strategies may not sufficiently address heat loss of the exhaust gas to exhaust components.
The inventors have recognized that one source of heat loss for exhaust gas is due to a rotational rate of the exhaust gas increasing as it travels to the exhaust catalyst. The rotational rate of the exhaust gas increases downstream of a turbine in engines that include turbochargers due to free rotational movement of the turbine. This free rotational movement of the turbine causes a rotational rate of the exhaust gas to increase as the exhaust gas flows through the turbine to the exhaust catalyst. The increased rotation of the exhaust gas as the exhaust gas travels axially downstream towards the exhaust catalyst increases a travel time or residency time of the exhaust gas from the exhaust manifold to the exhaust catalyst. Thus, an amount of contact between the exhaust gas and the walls of the exhaust conduit is increased, and an increased amount of heat loss from the exhaust gas to the exhaust conduit may occur prior to the exhaust gas reaching the exhaust catalyst.
In one example, the issues described above may be addressed by a method that includes, responsive to engine cold-start conditions, inhibiting movement of a shaft of a turbocharger via a shaft locking mechanism. The shaft locking mechanism may be a passive or an active shaft locking mechanism. Using a shaft locking mechanism as opposed to relying only on waste-gate position alone to control rotation of the turbocharger may be advantageous. For example, a shaft locking mechanism may be more accurate in controlling a rotational speed of a turbocharger shaft and/or may have a greater impact on flow rotation than waste-gate adjustments. The shaft locking may be provided in place of, or in addition to, waste-gate adjustments, if available.
By inhibiting a rotation of the turbocharger, the technical effect of reducing a rotational rate of exhaust gas spiraling through the exhaust passage downstream of a turbine of the turbocharger may be achieved, and a heat loss of the exhaust gas may be reduced as the exhaust gas travels to the exhaust catalyst. This reduction in heat loss from the exhaust gas as it travels to the exhaust catalyst may expedite catalyst light-off and may lead to reduced emissions.
As mentioned above, the shaft locking mechanism may either be a passive shaft locking mechanism or an active shaft locking mechanism. A passive shaft locking mechanism may include a phase-transitioning material that stalls a turbocharger, or resists turbocharger rotation, when in a solid state. As a temperature of the phase-transitioning material increases, a viscosity of the phase-transitioning material may decrease. Once the viscosity of the phase-transitioning material decreases below a threshold viscosity, rotation of the turbocharger shaft may be enabled. A melting point temperature, as defined by substances included in the phase-transitioning material, of the phase-transitioning material may correspond to a temperature at which the catalyst may operate at a desired efficiency, in some examples. A passive shaft locking mechanism may be advantageous for withstanding the high temperature conditions of the turbocharger shaft without degradation.
An active shaft locking mechanism may include a pin that may be actuated to fit into a recess of the shaft locking mechanism to stall rotation of the shaft of the turbocharger. An active shaft locking mechanism may be advantageous for quickly stalling or enabling rotation of the turbocharger shaft responsive to certain conditions and varying the conditions, such as temperature, at which rotation is no longer blocked. For example, the shaft rotation may be stalled until precisely when it is desired to enable shaft rotation. In one example, an active shaft locking mechanism may enable the shaft to be stalled during engine cold-start conditions until reaching a first temperature to reduce exhaust gas rotation, leading to expedited catalyst light-off, but stalled until reaching a second, different (e.g., lower or higher) temperature during other starting conditions or engine re-starting conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.